YJ or yJ may refer to:

Arts and entertainment:

• Yahoo! Japan, a website
• Young Justice, a DC comic series
  • Young Justice (TV series), TV series aired on Cartoon Network
    • Young Justice: Legacy, video games based on the TV series
• Yakitate!! Japan, an anime
• Weekly Young Jump, a magazine

Other uses:

• Yoctojoule (yJ = 10⁻²⁴  J), an SI unit of energy
• Yottajoule (YJ = 10²⁴  J), an SI unit of energy
• Jeep Wrangler, an SUV
• Young Judaea, a Zionist youth movement